X-ray energy is used in a number of different fields for a variety of purposes, both commercial and experimental. X-rays are often generated by x-ray vacuum tubes, which are evacuated chambers within which a beam of high-energy electrons are directed to a metallic target anode. The interaction of the electrons and the target causes both broad-spectrum bremsstrahlung and characteristic x-rays due to inner electron shell excitation of the anode material.
In certain fields, such as x-ray diffraction, it is the quasi-monochromatic characteristic x-rays that are the useful portion of the x-ray energy emitted from the anode. X-rays of various energies can be generated by selection of an appropriate anode material. For example, anodes of chromium, cobalt, copper or molybdenum are often used.
One problem in the field of x-ray generation is that the process is inherently inefficient, and most of the electron beam energy is dissipated as heat. As the x-ray power is increased (by increasing the power of the electron beam), the temperature of the anode will eventually reach the melting point of the anode material. Once this point is reached, the anode material will rapidly evaporate into the vacuum of the tube, destroying both the anode and the tube. Naturally, this limits the x-ray flux that can be produced by the tube.
The problem with localized heating of anodes in higher-power x-ray generation systems has been addressed by using a rotating anode configuration in which the anode surface rotates rapidly to spread the incident heat load over a larger surface area. As the brightness of a rotating anode x-ray generator is proportional to the power loading on the anode, so it is often desirable to increase this power loading. But the corresponding heat acts as a limit to the brightness achievable, even when using a rotating anode.
A typical, conventional anode is shown in FIG. 1. A thin ring 12 is constructed of a target material, such as copper or molybdenum, which has a desired characteristic x-ray emission in response to electron bombardment. In this example the ring is part of a hollow cup that may be constructed entirely of the characteristic material. The cup is connected to a shaft 11, and together the cup and shaft make up a rotating portion of the anode. The cup/shaft combination is concentric with a stationary distributor, or stator, 13, and between them lays a gap through which a cooling fluid may pass. The fluid may be introduced through an inlet 21 and removed via an outlet 22.
A parameter for the maximum power load of the anode is the shaft speed ω multiplied by the radius R of the cup. Thus, increasing the performance of the generator can be done by increasing the rotation speed ω or by increasing the cup radius R. The cooling of the anode surface takes place by forced fluid convection at the inner diameter of the cup. With the cooling liquid inside, the pressure P on the inside of the anode cup may be represented as:
  P  =            1      2        ⁢          ρ      c        ⁢                  ω        2            ⁡              (                              R            1            2                    -                      R            0            2                          )            where ρc is the specific mass of the fluid, R1 is the inner radius of the cup and R1−R0 is the thickness of the fluid layer. In the case of the conventional anode, R0 will, in most cases, be zero. Typical values with water as a cooling fluid might be:ρc=1000 kg/m3; ω=628 rad/s; R1=0.045 m; R0=0 m; and P=4 bar
The material stresses and sealing problems caused by the internal pressure are a limiting factor for significant improvements in generator performance. Turbulent losses of the cooling liquid in the anode give undesirable high pressure for pumping this fluid through the anode. At the same time, the torque caused by the fluid on the inner diameter of the anode is a significant part of the total driving torque needed to spin the anode.
A “heat pipe” is a well-known heat transfer mechanism. The basic principle behind a heat pipe is based on a closed-cycle fluid phase change, as is demonstrated in FIG. 2. A coolant (A) evaporates at a hot end (i.e., “evaporator section”) of the heat pipe. The hot vapor (B) is transported to a cool end (i.e., “condenser section”) by buoyancy forces, where it then condenses. The condensed fluid is returned to the hot end by gravity, centripetal forces or capillary action, thereby completing the cycle. Heat pipes, in general, demonstrate extremely efficient thermal transfer with an effective thermal conductivity of up to 10,000 times that of copper.
Rotating anodes for x-ray generators that use a heat pipe principle have been shown in the art. These prior art designs use a coolant fluid in a sealed chamber of the anode that is in thermal contact with a target region to be cooled. The target region is along a periphery of a rotating chamber of the anode, and the fluid is kept in contact with that region via centripetal force. Heat from the target evaporates a portion of the fluid, and the vapor moves toward a rotational axis of the chamber by buoyancy forces. In this inner region is a condensing plate against which the coolant condenses, and is returned to the periphery of the chamber under centripetal force. A cooling fluid flows through a fluid path that is in thermal contact with the condensing plate on the outside of the chamber.